Living things began tracking the incremental passage of time long before the human-made clock lent its hands. As life grew in harmony with the sun’s daily march through the sky, and with the seasons, phases of the moon, tides, and other predictable environmental cycles, evolution ingrained biology with the timekeeping tools to keep a step ahead.
The accompanying article is excerpted and adapted from “The Inner Clock: Living in Sync With Our Circadian Rhythms,” by Lynne Peeples. (Riverhead Books, 352 pages.)
It gifted an ability to anticipate changes, rather than respond to them, and an internal nudge to do things when most advantageous and to avoid doing things when not so advantageous. Of course, that optimal timing depended on a species’ niche on the 24-hour clock. When mammals first arose, for example, they were nocturnal — most active during the hours that the dinosaurs slept. Now mammals occupy both their choice territories on a spinning planet and their preferred space on a rotating clock.
Timing is everything when it comes to seeking and digesting food, storing food, avoiding becoming food, dodging exposure to DNA-damaging ultraviolet radiation, and many more vital activities, such as navigating, migrating, and reproducing.
Take the Eudyptula minor, a tiny penguin species that lives on Phillip Island in Australia. The slate-blue plumaged seabird speed waddles from the ocean to burrow home at the same “sun time” each day — just after sunset. Finding that precise window between day and night maximizes the penguins’ fishing time, allows them enough light to see their way to their burrows, and minimizes the chances they become visible food along the way for nighttime predators, such as orcas, seabirds, and feral cats. An internal clock off by just 10 minutes could prove fatal, one source told me.
The island’s tourism industry capitalizes on this predictable “Penguin Parade.” A website lists approximate penguin arrival times for every month of the year and sells tickets to witness the spectacle. A higher ticket price grants visitors access to an underground viewing structure where they can watch the procession of waddlers at eye level. In October 2022, lucky visitors got to view a record-breaking 5,440 little penguins storm the shore and hurry home.
Timing is everything when it comes to seeking and digesting food, storing food, avoiding becoming food, and many more vital activities, such as navigating, migrating, and reproducing.
I doubt anyone would pay to see Trypanosoma brucei, the parasite that causes sleeping sickness. But its timely feats are extraordinary, too. The parasite, endemic in sub-Saharan Africa, leverages its time-tracking mechanism to carry out a life cycle that includes hitching rides on tsetse flies, traveling through the bloodstream of a human or other animal, and ultimately disrupting that host’s circadian clocks. Patients sleep at strange times of day — hence, the name given to the deadly disease.
An arguably more charming example is the honeybee waggle dance. After returning to the hive from a successful foraging trip to, say, a sunflower, a worker bee will appear to waggle excitedly. A closer look shows she forms figure eights, shaking her abdomen as she moves across the middle line between the top and bottom rounds of the “eight.” It is thought that the duration of the waggling indicates the distance to the flower. And the orientation of this wiggly line in relation to the sun tells her hive mates the direction of the delectable nectar. But the dance includes a twist. Because that relationship to the sun changes hour by hour, the bee must continually update her dance, or she’ll lead her hivemates astray.
This meticulous choreography would not be achievable without a circadian clock — just as the flowering plant needs its biological rhythm to take full advantage of the bees’ services.
For you and me, our clock network manifests as regular rhythms inside our bodies — such as the ebb and flow of hormones and rise and fall of blood pressure and heart rate — and in our behaviors. Clocks help our digestive and metabolic systems to gear up ahead of time to efficiently process a meal, assuming those meals arrive at similar times each day, and the skeletal muscles to fire at peak force when they are most needed.
Our strength generally maxes out around dusk. That’s probably when our ancestral hunters would have hauled home their harvest. Most of us still organize our schedules somewhat similarly to those early humans, who restricted activities such as hunting and gathering to daytime, when they were less likely to fall off a cliff or end up a meal for a nocturnal predator.
Around dusk is also when the circadian system instructs the pineal gland in the brain to begin releasing melatonin. This hormone tells the body that darkness has descended and, for us diurnal creatures, that it is time for rest. It doesn’t directly make you sleepy but sets in motion other physiological processes that do.
Exposure to environmental cues such as the alternation of light and dark as our planet pirouettes keeps living clocks closely tethered to the 24-hour day.
You can also likely thank your well-tuned clocks for slowing down your kidneys’ urine production and enlarging the storage capacity of your bladder so you can sleep through the night without getting up to pee, and for spurring your adrenal glands to pump cortisol to rev you up for the new day. Your sleep, mood, appetite, immune response, sex drive, and body temperature all wax and wane under direction of your circadian rhythms. The list goes on.
Exposure to environmental cues such as the alternation of light and dark as our planet pirouettes keeps living clocks closely tethered to the 24-hour day. That connection is crucial for a penguin, parasite, or person to do the right thing at the right time. Life-forms evolving away from the equator also picked up on how the sun’s daily arc varied across the year. Changing day lengths signaled changing seasons, which warned of changing hazards and priorities.
Circadian rhythms could serve as reliable calendars in addition to daily clocks. For a male humpback whale, decreased day length is believed to prompt its migration south to its winter breeding grounds and perhaps motivate it to begin singing for a mate. For an Arctic fox or mountain hare, the hint of a seasonal shift can trigger a color change. A hare’s coat begins transitioning from summer brown to winter white as the days shorten. But now, with the rapid pace of climate change, the snow is melting earlier in the spring. “That mismatch means you can be a completely white rabbit in a brown forest,” and your predators can find you, said Micaela Martinez, an infectious disease ecologist with WE ACT for Environmental Justice.
The same day-length swings will nudge goats and many other animals into breeding season. Some farmers now fool that biology by using artificial lights to raise fertility rates in the natural offseason. It’s a strategy reminiscent of the ancient Japanese custom of yogai, in which people tricked caged birds into reproductive maturity — and, therefore, singing — by artificially extending the daily duration of daylight.
Lynne Peeples is a science journalist and former staff reporter at The Huffington Post. Her writing has also appeared in The Guardian, Undark, Scientific American, Nature, and other publications.